EP-A-0 795 614 discloses an air separation apparatus that sends gases or liquids into a storage tank when the power consumption of an arc furnace is low. When the furnace is operating at full power, the air separation apparatus is placed in reduced operation and the gas requirements of the furnace are supplied from the storage tank.
Energy deregulation has introduced electrical energy cost volatility. During relatively short time periods (which are known only very shortly before they occur), within the next few hours, the price of electricity may be multiplied by a factor of 10 relative to the base price, or by even more over an even shorter time.
One method of reducing energy costs of an air separation apparatus is disclosed in U.S. Pat. No. 5,315,521.
The air separation units are units that consume a very large amount of electrical energy and, in order to respond to this volatility in electrical pricing and to take advantage of it, have no other solutions than to shut down or reduce their production, but the latter solution does not allow a substantial reduction in the electrical energy consumed. Conventionally, the units may keep the products within given specifications by lowering the production down to about 65% of the nominal flow rates with energy consumption then lying between about 70% of the nominal consumption (that is to say the energy consumption for 100% production).
To meet all the requirements of their customers in terms of oxygen, nitrogen and other products obtained from air gases, which requirements generally remain constant even during such periods of energy volatility, these gases are liquefied beforehand (which represents a certain extra expenditure of energy) when electrical energy is available at a low cost, and these liquids are vaporized during these same peak periods. This liquid therefore supplements or completely covers the customers' requirements.
The problem with shutting down an air separation apparatus during these peak periods is that the time to restart it after the peak period is lengthy, around 8 to 24 hours, depending on the length of the shutdown, during which period it is always necessary to vaporize liquid produced beforehand, thereby reducing the economic benefit of this approach. Another disadvantage of this approach is the deterioration of the machines and other components, these repeated start-ups introducing fatigue that limits the lifetime of the said equipment.
“Optimal Demand-Side Response to Electricity Spot Prices for Storage-Type Customers” by Daryanian et al., IEEE Transactions on Power Systems, Vol. 4, No. 3, August 1989 explains that, in an air separation unit, it is worthwhile to permanently operate the compressor at full load irrespective of the customer's requirements.